Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 73)

Section 3. CR5000 Measurement Details
3-19
3.6 Measurements Requiring AC Excitation
Some resistive sensors require AC excitation. These include electrolytic tilt
sensors, soil moisture blocks, water conductivity sensors and wetness sensing
grids. The use of DC excitation with these sensors can result in polarization,
which will cause an erroneous measurement, and may shift the calibration of
the sensor and/or lead to its rapid decay.
Other sensors like LVDTs (without built in electronics) require an AC
excitation because they rely on inductive coupling to provide a signal. DC
excitation would provide no output.
Any of the bridge measurements can reverse excitation polarity to provide AC
excitation and avoid ion polarization. The frequency of the excitation can be
determined by the delay and integration time used with the measurement. The
highest frequency possible is 5 kHz, the excitation is switched on and then
reversed 100 µs later when the first measurement is held and then is switched
off after another 100 µs when the second measurement is held (i.e., reverse the
excitation, 100 µs delay, no integration).
Influence of Ground Loop on Measurements
When measuring soil moisture blocks or water conductivity the potential exists
for a ground loop which can adversely affect the measurement. This ground
loop arises because the soil and water provide an alternate path for the
excitation to return to CR5000 ground, and can be represented by the model
diagrammed in Figure 3.6-1.
FIGURE 3.6-1. Model of Resistive Sensor with Ground Loop
In Figure 3.6-1, V
x
is the excitation voltage, R
f
is a fixed resistor, R
s
is the
sensor resistance, and R
G
is the resistance between the excited electrode and
CR5000 earth ground. With R
G
in the network, the measured signal is:
( ) V V
R
R R R R R
x
s
s f s f G
1
=
+ +
/
[3.6-1]
R
s
R
f
/R
G
is the source of error due to the ground loop. When R
G
is large the
equation reduces to the ideal. The geometry of the electrodes has a great effect
on the magnitude of this error. The Delmhorst gypsum block used in the 227
probe has two concentric cylindrical electrodes. The center electrode is used